Synchronous rectifying circuit

Abstract

A synchronous rectifying circuit for a switching power supply is disclosed which is arranged such that the necessity to provide a diode element in parallel with a rectifying transistor is eliminated so that a highly efficient and stable operation can be achieved. "On" drive signal of a switching transistor (Q1) is detected by a first limiter circuit (6), and a flip-flop circuit (5) causes a rectifying transistor (Q2) to be turned off in response to output of the first limiter circuit (6); turning-off operation of the switching transistor (Q1) is detected by a second limiter circuit (7), and the flip-flop circuit (5) causes the rectifying transistor (Q2) to be turned on in response to output of the second limiter circuit (7). The necessity to provide a diode element in parallel with the rectifying transistor is eliminated. Stable operation can be performed even if the switching element has an indefinite operation delay time. An enhanced efficiency can be achieved.

We claim: 
     
       1. A synchronous rectifying circuit for use with a switching power supply using a transistor as rectifying element, the rectifying transistor being arranged to be driven in synchronism with switching operation of switching element, said synchronous rectifying circuit comprising: a first limiter circuit for detecting drive signal of said switching element;   a second limiter circuit for detecting operation of said switching element; and   a flip-flop circuit for causing said rectifying transistor to be turned on or off in response to output signals of said first and second limiter circuits,   wherein drive signal for turning on said switching element is detected, and said rectifying transistor is turned off in response thereto; and the turning-off of said switching clement is detected, and said rectifying transistor is turned on in response thereto.   
     
     
       2. A synchronous rectifying circuit according to claim 1, wherein said rectifying transistor comprises a MOS FET. 
     
     
       3. A synchronous rectifying circuit according to claim 1, wherein each of said limiter circuits comprises a differentiating circuit receiving an input signal, and a transistor switch adapted to be driven by said differentiating circuit. 
     
     
       4. A synchronous rectifying circuit for use with a switching power supply using a transistor as rectifying element, the rectifying transistor being arranged to be driven in synchronism with switching operation of switching element, said synchronous rectifying circuit comprising: a first limiter circuit comprising a differentiating circuit connected at input side to pulse output terminal of a control circuit for driving said switching element, and a transistor switch adapted to be operated in accordance with signal derived from said differentiating circuit;   a second limiter circuit comprising a differentiating circuit connected at input side to current output terminal of said switching element, and a transistor switch adapted to be operated in accordance with signal derived from said differentiating circuit;   a flip-flop circuit having a reset terminal connected to the current output terminal of the transistor switch of said first limiter circuit, and a set terminal connected to the current output terminal of the transistor switch of said second limiter circuit; and   said rectifying transistor comprising an N channel type MOS FET and adapted to be operated in accordance with signal derived from said flip-flop circuit.   
     
     
       5. A synchronous rectifying circuit according to claim 4, wherein two transistors of different conductivity types are connected with each other in complementary fashion; a common connection point for bases of said two transistors is coupled to the output terminal of said flip-flop circuit; and a common connection point for main current paths of said two transistors is connected to a control terminal of said rectifying transistor.